I’m using Unity 4.5, and I’m unable to run my compiled project due to the above error found in the log file. I’ve come across this error before, and I have determined a number of possible causes:
Two scripts with the same name - with or without different file extensions
A serializable class that derives from an abstract class, with that abstract class not marked serializable
Missing script errors in GameObjects or prefabs
MonoBehaviour/ScriptableObject derived classes not being in their own file with a name the same as the name of the class
Classes with the same name as an existing Unity class
I have checked and double checked all these conditions and re-imported all assets, to no avail. My project is currently a single scene with a reasonable amount of content, and an awful lot of code.
Okay, I think I found the error. It seems that certain types of serialized data became invalid in the transition from 4.3 to 4.5. The solution is to re-create the affected components, which typically have members of types that derive from generic classes.
EDIT: Also, nested classes. That may or may not have been a problem too
I also had this issue. For me it was a corrupted Prefab, even with switching to 2019. I recommend using version control software like Github, backing up your project, and then you can delete large swaths of your project (for example, I did one where I deleted scripts, that didn’t work, but then restored to the older version, deleted all prefabs, and the crash stopped.) From there I just had to rebuild prefabs… apparently meshes can also be corrupt. Hope this helps!